1. Field of the Invention
The present invention relates to an optical recording medium. More specifically, the present invention relates to an optical recording medium in which highly reliable recording and/or retrieving of optical information signals is performed.
2. Description of the Related Art
In recent years, in order to cope with the trend toward higher recording density required for optical recording media (hereinafter also simply referred to as “disks” or “optical disks”) technology for reducing the diameter of the spot of recording or retrieving light is being developed. Specifically, in the above-described technology, the diameter of the spot of recording or retrieving light is reduced by increasing the numerical aperture (NA) of an objective lens of an optical pickup. The objective lens is used for irradiating recording or retrieving light.
Moreover, in addition to increasing the NA of the objective lens, a study to thin an optically transparent layer of the optical recording medium through which the above-described recording or retrieving light passes is also being developed. If the thickness of the optically transparent layer is reduced, influences of aberration and birefringence caused by decreased tolerance of the tilt angle of a disk plane relative to the optical axis of the optical pickup are reduced.
In terms of thinning the optically transparent layer, a further study is being developed. Specifically, the layer configuration of the optical recording medium is made to be a structure in which a reflecting layer and a thin cover layer allowing recording or retrieving light to pass therethrough are sequentially stacked on a disk substrate having concavo-convex for optical information formed thereon. Further, an optical recording medium has been proposed in which, by means of the cover layer as an optically transparent layer, optical information is retrieved by irradiating retrieving light from the cover layer side. Such an optical recording medium is called a surface incident-type optical recording medium.
As a method of forming an optically transparent layer in a surface incident-type optical recording medium, a method has been proposed in which an optically transparent sheet is fixed to the surface of a substrate by use of an adhesive layer to be used as an optically transparent layer (e.g., refer to Patent Literature 1). Moreover, as a method of forming an optically transparent layer, a method has also been reported in which photocurable resin applied to the surface of a substrate by spin coating and cured to form an optically transparent layer (e.g., refer to Patent Literature 2).
On the other hand, with the development of a study to increase the NA of an objective lens, the distance (working distance) between the surface of an optical recording medium and an objective lens is reduced. This newly causes a problem. Specifically, if the optical recording medium is rotated at high speed while being held and fixed in a rotating device, the axial runout of the optical recording medium occurs. Due to the occurrence of the axial runout, recording and retrieving are prone to fail. Furthermore, since the working distance is small, the objective lens may come into contact with the surface of the optical recording medium to scratch the surface of the optical recording medium or the objective lens in the case where the axial runout is large.
As a method of reducing the influence of axial runout in a surface incident-type optical recording medium, for example, a method has been reported in which a clamp reference plane used for holding and fixing the optical recording medium in an external mechanism is provided on an optically transparent layer of the optical recording medium (e.g., refer to Patent Literature 3).    (Patent Literature 1) Japanese Unexamined Patent Publication No.  Hei 10(1998)-283683 (paragraph 0013, FIG. 6, and the like)    (Patent Literature 2) Japanese Unexamined Patent Publication No. Hei 9(1997)-161333 (paragraphs 0015 to 0021, FIG. 7, and the like)    (Patent Literature 3) Japanese Unexamined Patent Publication No. 2002-117584 (paragraph 0015, FIG. 1, and the like)
Incidentally, the recording and retrieving of the aforementioned optical recording medium provided with the optically transparent layer (cover layer) are generally performed by holding and fixing a clamp portion of the optical recording medium in a rotating device (external mechanism) of a read and write tester or the like and rotating the optical recording medium. At this time, in the case where the flatness of the clamp portion is poor, the axial runout of the optical recording medium is increased by rotating the optical recording medium at high speed in a state where the optical recording medium is fixed in the rotating device. For example, in a method in which a clamp reference plane is provided in the clamp portion on the surface of the optically transparent layer (cover layer), the axial runout of the disk is tend to be determined by the quality of the flatness of the optically transparent layer (cover layer) formed on the substrate, and further improvement is considered to be necessary.
Further, in the case where the optically transparent layer (cover layer) is formed by spin coating, there is a following problem: when a center cap used for obtaining the uniformity of the coating film thickness is removed after the completion of spin coating, uncured spin-coating layer near the edge of the center cap is prone to be made nonuniform, and therefore the flatness of the clamp portion is lowered.